Study On Construction Deformation Laws Of Deep Foundation Pit In Subway Station With Silty Clay Layer

Since twenty-first Century,with the rapid development of the economy and the scale of the city,more and more cities have built urban subway projects,of which the standard underground two layers of stations are buried more than 20 meters,the hydrologic and engineering geological conditions of the deep foundation pit of the station are complex,and the deformation control of the surrounding buildings should be higher.The first phase of Taiyuan Rail Transit Line 2 is the first rail traffic construction project in the city.The line is arranged parallel to the Fenhe River,the stratum belongs to the alluvial strata of the river,mainly with silty clay,with high groundwater level and rapid recharge.The hydrological and geological conditions are different from those in Beijing and Shanghai,and lack of construction examples and reliable experience,It is not clear about the deformation influence and laws of foundation pit excavation on the surrounding structure and surrounding environment to this kind of stratum.In this paper,take the deep foundation pit of a station in Taiyuan Rail Transit Line 2 as an example.In combination with field monitoring,numerical simulation and theoretical analysis,the deformation laws of the foundation pit itself and its surrounding environment is studied.The conclusions are as follows:(1)The deformation curve of the foundation pit retaining structure is “bow” shape,the maximum horizontal displacement is 0.09~0.14%H(H is the depth of pit excavation),the maximum deformation position is between 0.55 H and 0.8H;The surface settlement curve is groove shape,and the maximum value of surface subsidence is 0.106~0.139%H,the maximum horizontal distance from the edge of the pit is about 0.22~0.4H;the monitoring value of the supporting axial force is less than the design value,which is about 50% to 70% of the design value;With the increase of excavation depth,the uplift curve of the excavation face gradually changed from "arch" to "trapezium".(2)The excavation by stratified subsection method is smaller than that by the overall stratification method,and the horizontal deformation of the foundation pit retaining structure is reduced by 6.89%,the surface subsidence value is reduced by 12.67%,and the basement uplift value is basically the same.(3)The Longitudinal segment length and vertical layer thickness of soil excavation greatly affect the deformation of the foundation pit.The deformation of the retaining structure and the surface subsidence in the pit increase with the increase of the longitudinal segment length and the vertical layer thickness,and the base uplift value is not influenced by the longitudinal segment length and the vertical layer thickness.(4)The over excavation of earthwork has a significant influence on the deformation of the foundation pit.The over excavation of the earthwork has the greatest impact on the third supporting positions,followed by the second supporting positions,and the influence of the first supporting position is limited.(5)The pre-axial force can effectively control the deformation of the foundation pit.With the increase of the pre-axial force the deformation of the retaining structure and the surface subsidence are obviously reduced.,but the rate gradually decreases.The deformation of the 100% pre-axial loading condition is 55.09% less than that of no-preloading working condition,and the surface subsidence is reduced by 60.41%.